Distant Radio Galaxies Research Articles

Masses of distant radio galaxies: A coherent view for evolution and origin

AbstractWe hereafter aim to present mass estimates and main parameters of distant powerful radio sources, respectively for evolved stars, ionized gas, and dust. Observations and models are consistent with a scenario where large masses are accumulated whithin extremely short time scales < 1 Gyr at redshifts z > 2: a strong constraint for large‐scale structure models. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Clustering of Star‐forming Galaxies Near a Radio Galaxy atz = 5.2

We present HST ACS observations of the most distant radio galaxy known, TN J0924-2201 at z = 5.2. This radio galaxy has six spectroscopically confirmed Ly?-emitting companion galaxies and appears to lie within an overdense region. The radio galaxy is marginally resolved in i775 and z850, showing continuum emission aligned with the radio axis, similar to what is observed for lower redshift radio galaxies. Both the half-light radius and the UV star formation rate are comparable to the typical values found for Lyman break galaxies at z ~ 4-5. The Ly? emitters are sub-L* galaxies, with deduced star formation rates of 1-10 M? yr-1. One of the Ly? emitters is only detected in Ly?. Based on the star formation rate of ~3 M? yr-1 calculated from Ly?, the lack of continuum emission could be explained if the galaxy is younger than ~2 Myr and is producing its first stars. Observations in V606i775z850 were used to identify additional Lyman break galaxies associated with this structure. In addition to the radio galaxy, there are 22 V606 break (z ~ 5) galaxies with z850 99%), based on a counts-in-cells analysis applied to the control field. The excess suggests that the V606 break objects are associated with a forming cluster around the radio galaxy.

A search for distant radio galaxies from SUMSS and NVSS -- II. Optical spectroscopy

This is the second in a series of papers presenting observations and results for a sample of 76 ultra-steep-spectrum (USS) radio sources in the southern hemisphere designed to find galaxies at high redshift. Here we focus on the optical spectroscopy program for 53 galaxies in the sample. We report 35 spectroscopic redshifts, based on observations with the Very Large Telescope (VLT), the New Technology Telescope (NTT) and the Australian National University's 2.3m telescope; they include five radio galaxies with z>3. Spectroscopic redshifts for the remaining 18 galaxies could not be confirmed: three are occulted by Galactic stars, eight show continuum emission but no discernible spectral lines, whilst the remaining seven galaxies are undetected in medium-deep VLT integrations. The latter are either at very high redshift (z >~7) or heavily obscured by dust. A discussion of the efficiency of the USS technique is presented. Based on the similar space density of z>3 radio galaxies in our sample compared with other USS-selected samples, we argue that USS selection at 843-1400 MHz is an efficient and reliable technique for finding distant radio galaxies.

Diffuse polarized emission associated with the Perseus cluster

We report on full-polarization radio observations of the Perseus cluster (Abell 426) using the Westerbork Synthesis Radio Telescope (WSRT) at wavelengths from 81-95 cm. We detect faint, very extended polarized emission throughout the cluster region. We have employed a novel technique, Rotation Measure synthesis (Brentjens & de Bruyn, 2005, A& A, 441, 1217) to unravel the polarization properties of the emission across the full field of view. We detect polarized emission over a wide range of RM from about 0 to 90 radm(-2). Low RM emission (RM 30 rad m(-2) on the other hand, shows organized structures on scales up to a degree and displays rapidly fluctuating polarization angles on scales of the synthesized beam. A Galactic foreground interpretation for the high RM emission can not be ruled out, but appears extremely implausible. WSRT observations at 21 cm of the RM of a dozen discrete sources surrounding the Perseus cluster indicate a smooth large-scale gradient in the Galactic foreground RM. The diffuse structures have a clear excess RM of about 40 rad m(-2) relative to these distant radio galaxies. This excess Faraday depth, the generally good spatial association with the cluster and the different morphology of the high RM emission, compared to the genuine Galactic foreground emission, all point to an association of the high RM emission with the Perseus cluster. The polarized emission reaches typical surface brightness levels of 0.5-1 mJy per 2' x 3' beam and must be rather highly polarized (greater than or similar to 20%). Due to dynamic range limitations and lack of sensitivity to large-scale structure we have not yet detected the corresponding total intensity. Most of the polarized emission, located at distances of about 1 degrees from the cluster centre, appears too bright, by about 1-2 orders of magnitude, to be explainable as Thomson scattered emission of the central radio source off the thermal electrons in the cluster. However, this remains a viable explanation for the highly polarized 21 cm emission from the inner 10'-20' and part of the 81-95 cm emission. The bulk of the emission associated with the Perseus cluster may instead be related to buoyant bubbles of relativistic plasma, probably relics from still active or now dormant AGN within the cluster. A lenticular shaped structure, referred to as the lens, and measuring 0.5-1 Mpc is strikingly similar to the structures predicted b En beta lin et al. ( 1998, A& A, 332, 395). At the western edge of the cluster, we detect very long, linear structures that may be related to shocks caused by infall of gas into the Perseus cluster along the Perseus-Pisces filamentary structure of the cosmic web.

Simulation of Astrophysical Jets: Collimation and Expansion into Radio Lobes

Jets in active galactic nuclei are created in accretion disks around supermassive black holes, and they can retain their collimation up to the order of a megaparsec. To understand the physical causes of this remarkable collimation over such large distances is a great challenge and could lead to important new insights in the physics of magnetoplasmas. In the present work we take a two-pronged approach. First, we simulate the behavior of jets using a new, recently developed, mathematical model. Second, we analyze the observational evidence from the well-defined jet of the relatively distant radio galaxy 3C 303. We derive new constraints on the plasma properties of the jet. The final goal is to determine the ability of MHD models to describe the collimation of extragalactic jets, their physical nature, and their role in formation of radio lobes.

Constraining Van der Waals quintessence with observations

We discuss an alternative approach to quintessence, modifying the usual equation of stateof the cosmological fluid in order to see whether going further than the approximation ofperfect fluid allows one to better reproduce the available data. We consider a cosmologicalmodel comprising only two fluids, namely baryons (modelled as dust) and darkmatter with a Van der Waals equation of state. First, the general features of themodel are presented and then the evolution of the energy density, the Hubbleparameter and the scale factor are determined showing that it is possible to obtainaccelerated expansion by suitably choosing the model parameters. We use thedata on the dimensionless coordinate distances to type Ia supernovae and distantradio galaxies to see whether Van der Waals quintessence is viable for explainingdark energy and for constraining its parameters. We then compare the modelpredictions with the estimated age of the universe and the position of the first threepeaks of the anisotropy spectrum of the cosmic microwave background radiation.

CO (1-0) and CO (5-4) Observations of the Most Distant Known Radio Galaxy at z = 5.2

Using the Australia Telescope Compact Array, we have detected CO (1-0) and CO (5-4) from TN J0924-2201 at z = 5.2, the most distant radio galaxy known to date. This is the second highest redshift detection of CO published so far. The CO (1-0) line emission is 250-400 km s-1 wide with a peak flux density of 520 ± 115 μJy beam-1, while the CO (5-4) line emission is 200-300 km s-1 wide with a peak flux density of 7.8 ± 2.7 mJy beam-1. Both transitions are spatially unresolved, but there is marginal evidence for spatial offsets between the CO and the host galaxy; the CO (1-0) is located 28 ± 11 kpc (45 ± 17) north of the radio galaxy, while the CO (5-4) is located 18 ± 8 kpc (28 ± 12) south of the radio galaxy. Higher spatial resolution observations are required to determine the reality of these offsets. Our result is the second detection of CO in a high-redshift galaxy without preselection based on a massive dust content.

Mapping the Local and Distant Universe with SUMSS

The Sydney University Molonglo Sky Survey (SUMSS) is a radio continuum imaging survey of the southern sky at 843 MHz, with similar sensitivity and resolution to the northern NRAO VLA Sky Survey (NVSS). We have combined radio data from SUMSS and NVSS with optical spectra from the 2dF Galaxy Redshift Survey and 6dF Galaxy Survey to study the space distribution and properties of both AGN and star-forming galaxies in the local universe (redshift range 0 < z < 0.3). We also discuss new results on radio-source clustering in the more distant universe (z ∼ 1) and present the first results from a search for the most distant (z > 3) southern radio galaxies.

Extended X-ray emission at high redshifts: radio galaxies versus clusters

Most old distant radio galaxies should be extended X-ray sources due to inverse Compton scattering of Cosmic Microwave Background (CMB) photons. Such sources can be an important component in X-ray surveys for high redshift clusters, due to the increase with redshift of both the CMB energy density and the radio source number density. We estimate a lower limit to the space density of such sources and show that inverse Compton scattered emission may dominate above redshifts of one and X-ray luminosities of 10^44 erg/s, with a space density of radio galaxies > 10^-8 Mpc^-3. The X-ray sources may last longer than the radio emission and so need not be associated with what is seen to be a currently active radio galaxy.

Dust and star formation in distant radio galaxies

In this thesis I discuss recent observations of distant (z > 2) radio galaxies. There is strong evidence that radio galaxies are the progenitors of the brightest cluster ellipticals and are among the most luminous and massive galaxies at any epoch, allowing relatively detailed studies of their formation process out to large distances. Submillimeter emission has been detected in many, implying star formation rates of order 1000 Msun/yr over scales of tens of kpc, consistent with a scenario in which these galaxies are currently forming the bulk of their eventual stellar population. Many radio galaxies also host giant (~ 150 kpc) emission line nebulae. We obtaind very deep narrow-band Ly-a images for a select few of these. The observations show spectacular detail with a wealth of morphological structure, such as extended (~ 80 kpc) filamentary structures, giving insight in feedback processes during their formation. I discuss the possible origin and the ionization source of these halos in connection with the sub-mm results, and discuss possible implications for theories of galaxy formation.

The radio galaxy K-z relation: The $\mathsf{10^{12}}~$M\odot mass limit

The narrow K-z relation of powerful radio galaxies in the Hubble K-band diagram is often attributed to the stellar populations of massive elliptical galaxies. Because it extends over a large range of redshifts (), it is difficult to estimate masses at high redshifts by taking into account galaxy evolution. In the present paper, we propose to estimate the stellar masses of galaxies using the galaxy evolution model PÉGASE. We use star formation scenarios that successfully fit faint galaxy counts as well as galaxy templates. These scenarios also predict spectra at higher z, used to estimate valid photometric redshifts. The baryonic mass of the initial gas cloud is then derived. The K-z relation is remarkably reproduced by our evolutionary scenario for elliptical galaxies of baryonic mass , at all z up to . is also the maximum mass limit of all types of galaxies. Using another initial mass function (IMF), even a top-heavy one, does not alter our conclusions. The high value of observed at implies that massive clouds were already formed at early epochs. We also find that the limit is similar to the critical mass of a self-gravitating cloud regulated by cooling (Rees & Ostriker [CITE]; Silk [CITE]). Moreover, the critical size is remarkably close to the typical diameter of Lyα haloes surrounding distant radio galaxies. This confirms the validity of the method of baryonic mass determination based on the K-band luminosity. A puzzling question that remains to be answered is the short time-scale of mass-accumulation required to form such massive galaxies at . We discuss the dispersion of the K-z relation in terms of uncertainties on the mass limit. The link between the presence of the active nucleus and a large stellar mass is also discussed.

A search for distant radio galaxies from SUMSS and NVSS -- I. Sample definition, radio and K-band imaging

We present the first results from a pilot study to search for distant radio galaxies in the southern hemisphere (δ 15 mJy. We observed 71 sources without bright optical or near-infrared counterparts at 1.385 GHz with the ATCA, providing ∼5 arcsec resolution images and subarcsec positional accuracy. To identify their host galaxies, we obtained near-infrared K-band images with IRIS2 at the AAT and Soft at the NTT. We identify 92 per cent of the USS sources down to K ∼ 20.5. The SUMSS-NVSS USS sources have a surface density more than 4 times higher than USS sources selected at lower frequencies. This is due to the higher effective selection frequency, and the well-matched resolutions of both surveys constructed using the same source fitting algorithm. The scattering of α > -1.3 sources into the USS sample due to spectral index uncertainties can account for only 35 per cent of the observed USS sources. Because our sample appears to contain a similar fraction of very distant (z > 3) galaxies, selecting USS sources from SUMSS-NVSS should allow us to identify large numbers of massive galaxies at high redshift.

A large population of 'Lyman-break' galaxies in a protocluster at redshift z approximately 4.1.

The most massive galaxies and the richest clusters are believed to have emerged from regions with the largest enhancements of mass density relative to the surrounding space. Distant radio galaxies may pinpoint the locations of the ancestors of rich clusters, because they are massive systems associated with 'overdensities' of galaxies that are bright in the Lyman-alpha line of hydrogen. A powerful technique for detecting high-redshift galaxies is to search for the characteristic 'Lyman break' feature in the galaxy colour, at wavelengths just shortwards of Lyalpha, which is due to absorption of radiation from the galaxy by the intervening intergalactic medium. Here we report multicolour imaging of the most distant candidate protocluster, TN J1338-1942 at a redshift z approximately 4.1. We find a large number of objects with the characteristic colours of galaxies at that redshift, and we show that this excess is concentrated around the targeted dominant radio galaxy. Our data therefore indicate that TN J1338-1942 is indeed the most distant cluster progenitor of a rich local cluster, and that galaxy clusters began forming when the Universe was only ten per cent of its present age.

3D spectroscopy of the radio galaxy 3C171 ( z=0.238) with OASIS/CFHT

Distant radio galaxies, observed up to the highest redshifts ( z≃6), are characterized by intense emission lines. Measuring emission line ratios bring clues on ionizing processes. When massive stars are ionizing sources, they give the information on the primeval star formation activity. However analyses are difficult for several reasons. Surface brightness and spatial resolution are rapidly decreasing with redshift. Moreover various emission processes (photoionization by the central AGN, shocks due to jet-cloud interactions or the coupling of ionized gas with radio emitting plasmas) may partly obliterate the photoionization by massive stars. The contribution of star formation at high redshift can only be derived after substraction of other contributing processes. The solution is to locally disentangle the various contributions to emission lines with the help of integral field spectroscopy. We firstly select, with the help of models, the emission line ratio diagnostics able to separate ionizing processes by reproducing at best the observations. Then we observe distant radio galaxies by using integral field units (IFUs) to build maps of emission line ratios. Models are applied by zones, allowing a comparative interpretation between zones. Results give the main sources of ionization in each zone. We present results of 3D spectroscopy on the FR-II radio galaxy 3C171 ( z=0.238) observed with OASIS/CFHT. The H α emission line map represents a typical ionized cocoon, as predicted by theory. The [OIII] λ5007/H β map shows some striking features. Preliminary results from the zone by zone analysis of the [OIII] λ5007/H β map with our recent modelling ( Moy and Rocca-Volmerange, 2002) is also highlighted.

A deep Chandra observation of 3C294

We present the results of a ∼200 ks deep look at the powerful distant radio galaxy 3C294 with the Chandra X-ray telescope.

Spectroscopy of RC sources

The results of spectroscopic observations of the host galaxies of objects in the RC catalog (the “Big Trio” program) obtained using the new SCORPIO spectrograph of the Special Astrophysical Observatory are presented. The spectroscopic redshifts of the objects are compared with their photometric color redshifts, and the errors in the latter are estimated. Based on BV RI observations obtained on the 6-m telescope of the SAO, the errors for the population of powerful radio galaxies are close to those found previously for radio quiet galaxies (about 10–20%). The detection of Ly α in the B filter in RC 1626+0448 is confirmed. This object is the second spectrally studied FR II radio source from the RC catalog to have a redshift z>2.5. Star formation in its host galaxy began at a redshift z>3.3. This first use of the new SCORPIO spectrograph demonstrates its promise for studies of very distant steep-spectrum radio galaxies brighter than 23m–24m in V.

Color redshifts and the age of the stellar population of distant RC radio galaxies

BV RI data are presented for the majority of steep-spectrum objects in the RC catalog with mR<23.5m. Previously developed programs are applied to these data to estimate the redshifts and ages of the stellar systems of the host galaxies. Applying this program to the color data (BV RI JHK) for distant radio galaxies with spectroscopic redshifts indicates that this approach provides accurate estimates of the redshifts of such radio galaxies, close to those obtained using field galaxies (∼20%). The age estimates are much less trustworthy, but a lower limit to the ages of objects that are not very distant (z<1.5) can be determined with certainty. We have identi fied several galaxies whose formal ages exceed the age of the Universe at the corresponding z in simple Cold Dark Matter models for the Universe. The possibility of using such objects to elucidate the role of “dark energy” is discussed. This paradox disappears in models with cosmological constants (Λ terms) equal to 0.6–0.8.

Spatially Resolved High-Ionization Nebulae in a High-Redshift Radio Galaxy: Evidence for Shock Ionization and Photoionization

We present high-resolution, long-slit spectroscopic data obtained from the W. M. Keck I Telescope of one of the most distant radio galaxies reported to date: MG 2141+192 (B1950, z = 3.594 ± 0.003). Line emission from Lyα λ1215.7, C IV λ1549.0, and He II λ1640.5 spans an unusually large spatial extent (~120 kpc), continuing well beyond the radio lobes. We also report a large velocity field of multiple components spanning an FWZI of ~3200 km s-1. Such spectacular extended high-ionization emission of C IV and He II is rare in the published literature. The optical host galaxy is aligned with the radio axis (ΔP.A. ≈ 5°) of this Fanaroff-Riley type II radio galaxy, as is commonly observed in powerful and/or high-redshift radio galaxies. The line-emission nebulae have dissimilar velocity fields, suggesting that their origin is unlikely to be dominated by radiation from a reflected broadline region, but rather that from radiating turbulent gas. Our spatially resolved data provide information to probe the underlying energetics that power the rest-frame UV line emission. Previous modeling of radio galaxy ionization nebulae generally considered two scenarios: photoionization from a central active nucleus and shock ionization possibly resulting from radio jet interactions with the host galaxy environs. Integrated flux ratios of UV lines have frequently been used to distinguish between models. This work spatially resolves high-ionization lines to allow registered two-dimensional line ratio maps as an additional diagnostic. Comparing these maps with available shock and photoionization models reveals evidence for separate shock-ionized and photoionized emission regions. Our results imply that multiple ionization mechanisms may contribute to the line emission in active galactic nuclei, which must be carefully taken into consideration, particularly in analyses relying on total line fluxes.

COSMIC BIREFRINGENCE WITHIN THE FRAMEWORK OF HETEROTIC STRING THEORY

Low energy string theory predicts the existence of an axion field which can lead to cosmic birefringence. We solve the electromagnetic wave equations in the presence of such an axion and a dilaton field in order to determine their effect on the polarization of light. We find that the presence of dilaton field leads to a nontrivial modification of the final result. We comment on the possibility of discovering such an effect by observations of radio wave polarizations from distant radio galaxies and quasars. We have also determined the limits on the string theory parameters that are imposed by the current radio polarization data.

3D Spectroscopy in Distant Radio Galaxies

Star formation rates in distant radio galaxies are crucial for understanding the evolution of galaxies. Most processes at works in these targets observed at the most remote epochs are directly or not related to primeval sources of star formation: interplays of starbursts and AGNs, interactions of radio jets with environmental clouds. cooling flows, metal enrichment and dust distributions. Signatures of typical star formation (emission line ratios, equivalent widths, stellar continua) are observed in distant galaxies. However in most cases, they are superimposed to signatures of other processes (shocks, AGN light scattering). in particular at high z with the decrease of angular resolution and surface brightness. A larger variety of environmental components (lobes, jets, clouds and nucleus) simultaneously contribute to the global emission. So that to separate the varions emission processes becomes rapidly essential with higher distances. The 3D spectroscopy is unique to disentangle the various physical processes triggering star formation at high redshifts. Shocks and regions photoionized by the central AGN are identified from their typical features and may be localized in the various zones of the radio galaxy extended to its environment. We present results of a long term observationnal prograin with the successive Integral Field Units (IFU) TIGER and OASIS at CFHT. The present sample consists of three galaxies of increasing z: 3C 171 {z = 0.238), 3C 435A (z = 0.470) and 4C 41.17 (z = 3.8). Line ratios, continua and velocity fields are locallv measured. Interpretations of emission line ratios, velocity fields, equivalent widths, are possible with a coupled code based on CLOUDY and MAPPINGSIII results, as proposed by Moy et al. (2000, 2001). Stellar and nebular continua are interpretcd with the help of our evolutionary synthesis model PEGASE2. Ail models take into account metallicity effects.